1. Field of the Invention
This invention relates to a liquid jet recording head which performs recording by jetting a liquid to form flying liquid droplets.
2. Description of the Prior Art
Ink jet recording methods (liquid jet recording methods) are recently attracting attention for such advantages that generation of noise during recording is negligibly very small, that high speed recording is possible and also that recording can be done on so called plain paper without need of the special treatment of fixing.
Among them, the liquid jet recording method as disclosed in, for example, Japanese Laid-open Patent Application No. 51837/1979, Deutsche Offenlegungsschrift (DOLS) 2843064 has a specific feature different from other liquid jet recording methods in that the driving force for discharging liquid droplets is obtained by permitting heat energy to act on a liquid.
That is to say, according to the recording method disclosed in the above patent publications, liquid which has received action of heat energy undergoes a change in state accompanied with an abrupt increase of volume, and through the acting force based on the change in state is discharged liquid through the orifice at the tip end of the recording head section to be formed into flying liquid droplets, which liquid droplets are attached onto a material to be recorded, thereby effecting recording thereon.
In particular, the liquid jet recording method disclosed in the publication of DOLS No. 2843064 is not only applicable very effectively for the so called drop-on demand recording method, but also can easily be embodied into a recording head in which the recording head portion is made into a high density multi-orifice of full line type, thus being capable of giving images of high resolution and high quality at high speed.
The recording head section of the device to be applied for the above-mentioned method has a liquid discharging portion having an orifice provided for discharging liquid and a liquid pathway, which is connected to the orifice and has a heat acting portion at which heat energy acts on liquid for discharging liquid droplets, and an electro-thermal transducer as a means for generating heat energy. And, the electrothermal transducer has a pair of electrodes and a heat generating resistance layer which is connected to these electrodes and has a region for heat generation (heat generating portion) between these electrodes.
A typical example exhibiting the structure of such a liquid jet recording head is shown in FIG. 1(a) and FIG. 1(b). FIG. 1(a) is the front view of a liquid jet recording head as seen from the orifice side, and FIG. 1(b) is a partial sectional view of FIG. 1(a) when cut along a dot and dash line X - Y.
The recording head 100 has a structure having formed orifices 104 and liquid discharging sections 105 by bonding a grooved plate 103 provided with a certain number of grooves of certain width and depth at a predetermined line density to a substrate 102 provided on its surface with an electro-thermal transducer 101 so as to cover over the surface of the substrate 102. In the case of the recording head as shown in the FIG. 1, it is shown as having a plural number of orifices 104. Of course, the present invention is not limited to such embodiments, but also a recording head with a single orifice is included in the scope of the present invention.
The liquid discharging section 105 has an orifice 104 for discharging liquid at its terminal end and a heat acting portion 106, which is the place where heat energy generated from an electro-thermal transducer 101 acts on liquid to generate a droplet and cause abrupt change in state through expansion and shrinkage of its volume.
The heat acting portion 106 is positioned above the heat generating portion 107 of the electrothermal transducer 101 and has a heat acting face 108 in its bottom surface to be contacted with the liquid at the heat generating portion 107.
The heat generating portion 107 is constituted of a lower layer 109, a heat generating resistance layer 110 provided on the lower layer 109 and an upper layer 111 provided on the heat generating resistance layer 110. The heat generating resistance layer 110 is provided on its surface with electrodes 112 and 113 for passage of current to the layer 110. The electrode 112 is common to the heat generating portions of the respective liquid discharging sections, while the electrode 113 is a selective electrode by selecting the heat generating portion of each liquid discharging section for heat generation and is provided along the liquid pathway of the liquid discharging section.
The upper layer 111 has the function of separating the heat generating resistance layer 110 from the liquid filling the liquid pathway of the liquid discharging section for protection of the heat generating resistance layer 110 chemically or physically against the liquid employed at the heat generating portion 107, and also has the protective function for the heat generating resistance layer 110 to prevent short-circuit through the liquid between the electrodes 112 and 113. The upper layer 111 also serves to be under charge of preventing electrical leakage between adjacent electrodes. In particular, prevention of electrical leakage are between the respective selective electrodes or prevention of electric corrosion, which will occur by passage of current between the electrode under each liquid pathway and the liquid which may happen to be contacted for some reason, is important and for this purpose the upper layer 111 having such a protective function is provided at least on the electrode existing under the liquid pathway.
Further, the liquid pathway provided at each liquid discharging section is connected upstream thereof to the common liquid chamber (not shown in the Figure) for storage of the liquid to be supplied to said liquid pathway, and the electrode connected to the electro-thermal transducer provided at each liquid discharging section is generally provided for convenience in designing so that it may pass beneath the aforesaid common liquid chamber on the side upstream of the heat acting portion. Accordingly, it is generally practiced to provide the upper layer as described above even at this portion in order to prevent contact between the electrode and the liquid.
Whereas, the above-mentioned upper layer is required to have characteristics which are different depending on the place at which it is to be provided. For example, at the heat generating portion 107, it is required to be excellent in (1) heat resistance, (2) liquid resistance, (3) liquid penetration prevention, (4) thermal conductivity, (5) antioxidation property, (6) dielectric property and (7) breaking resistance, while in regions other than the heat generating portion 107, it is required to be excellent sufficiently in liquid penetration prevention, liquid resistance and breaking resistance, although these may be somewhat alleviated depending on the thermal conditions.
However, there is nowadays no material for constituting the upper layer which can satisfy all of the above characteristics (1) to (7) as desired, and under the present situation, some of the characteristics (1) to (7) are placed under alleviated requirements. That is to say, choice of material in the heat generating portion is done with preference posed on (1), (4) and (5), while in other portions than the heat generating portion 107, for example, the electrode portion, choice of material is done with preference posed on (2), (3) and (7), thus forming the upper layers with the use of corresponding materials on the respective regional faces.
On the other hand, as different from these, in the case of a multi-orifice type liquid jet recording head, formation of respective layers and partial removal of the layers formed are conducted repeatedly on a substrate in the manufacturing step for the purpose of forming a number of minute electro-thermal transducer simultaneously on the substrate. At the stage when the upper layer is formed, the surface on which the upper layer is to be formed is formed in minute concavo-convex shape with slab wedge portion (stepped portion), and therefore the step coverage characteristic of the upper layer at this stepped portion becomes important. In short, if the step coverage characteristic at this stepped portion is not enough, penetration of liquid will occur at that portion, whereby electric corrosion or breaking of dielectric strength may be induced. Also, when the upper layer is susceptible to occurrence of failures at a probability which is not low during manufacturing, liquid will penetrate through the failures to cause markedly lowering of life of the electro-thermal transducer.
For the reasons as mentioned above, the upper layer is required to be good in step coverage characteristic at the stepped portion, low in probability of occurrence of pinholes in the layer formed or if formed at all, are so small as to be negligible.
However, in the prior art, no liquid jet recording head has been proposed, which can satisfy all of these requirements and is excellent in overall use durability.